Histological and ultrastructural studies of the hearts of dogs sacrificed 18 to 26 days after i.p. inoculation with 4x105 blood forms of the 12 SF strain of T. cruzi/kg of body weight disclosed myocarditis characterized by parasitic invasion of some myocytes, damage and necrosis of nonparasitized myocytes, and interstitial infiltration by mononuclear cells. Nonparasitized myocytes showed alterations ranging from mild edema to severe myocytolysis. These changes often were accompanied by contacts of myocytes with lymphocytes (both granular and agranular) and macrophages. Our findings suggest that immune effector cells play a major role in the pathogenesis of the myocyte damage and the microangiopathy in acute Chagas' disease.